When a particle meets its own antiparticle, they annihilate, converting mass into energy. However, interactions between different particle-antiparticle pairs, such as a proton and an antineutron, can lead to various outcomes depending on the energy levels involved. At high energies, particles can create new particle-antiparticle pairs and undergo complex interactions, including the exchange of constituents. The conservation of energy and momentum plays a crucial role in these interactions, making the outcomes unpredictable. Overall, while annihilation is possible in certain conditions, many interactions will not result in annihilation, especially at lower energies.